Sean M Carroll on Origin of the Universe & the Arrow of Time

I will be talking about quantum a little bit but what I really want to get across is the idea that in our everyday lives in the days we go through and the things we experience in the world around us we're actually feeling the effects of the much larger universe in which we lived in particular what happens to us in our kitchens the fact that we can take an egg and turn it into an omelet but it's very hard to take an omelet and turn it back into an egg is a remnant of the fact that we live after the Big Bang and the Big Bang was a very special event so what's going on here is as you see the arrow of time the fact that time has a direction the fact that we all agree on the difference between yesterday and tomorrow as far as we know everywhere in the universe that we can observe we have the same feeling for which day is yesterday which day will be tomorrow and that's something that shows up in our everyday lives that helps us get to the festival events on time and so forth but it turns out that there's a profound mystery lurking here there's a lot about the arrow of time the physicists do understand there's one very important thing that they don't understand that's the origin of it all it turns out that the reason why yesterday is different from tomorrow is because the day before yesterday was different than yesterday and that's because the day before the day before yesterday was different than that and you can go all the way back 14 billion years to the Big Bang the fact then you can take an egg and make an omelet but not an omelet and make an egg is because of the Big Bang that's the message I want to get across in this talk today but let's start with thinking about time more generally we all know about time we use it to meet friends at the right moment we use it to get to work on time we judge our lives by it but there's different aspects to it when you start talking about defining time you get into trouble very quickly because we use the same word to mean different things so I just want to separate out two important notions of time one is that things repeat themselves things are predictable in nature they happen over and over again in a way that you can be confident it's going to be the same every time you can't even say these sentences without using the word time some way or another you know the Sun is going to rise in the east every single day my be foggy you might not be able to see it but it's going to happen that's a repetitive synchronization and that's what lets us measure the passage of time but the more important feature of time is that some things don't happen over and over again some things only happen in one direction or even if they don't only happen in one direction they're never quite the same when they come back to where they started and it's this aspect of change is what gives time its arrow and the interesting thing is that that arrow of time the difference between the past in the future is nowhere to be found in the fundamental deep down laws of physics the fundamental laws of physics are all about repetition and cyclic Ness and things happening over and over again but the world around us time has a direction what's going on there so it's the repetition that allows us to measure the passage of time and what I mean by this is if you try to define what you meant by certain number of minutes passing or a certain number of hours passing it would always be in terms of something else happening a certain number of times so we have a clock we know that every time the Earth revolves around rotates around its axis so the Sun comes up goes down then comes up again the hand on our clock is going to go around 24 times the minute hand on our clock the hour hand is going to go around twice because we've chosen to use twelve hours on our clocks but it's that predictability that makes it possible for us to measure time so I've drawn some pictures here if we go time running up from the bottom of the picture to the top you see one day then the next day then the next day and what happens is that in every interval of time certain things always happen the same number of times in one day the Earth rotates once around its axis if you have a clock pendulum rocking back and forth one second that's going to happen 864 thousand times every day the reason why quartz clocks are especially good at telling time is because there are little crystals made of quartz that vibrate very very predictably throughout technology was always a very difficult project to come up with a clock that was reliable that worked on the ocean for example a pendulum doesn't work very well when there are waves pushing you back and forth but the little quartz crystal always vibrates the state number of times per second no matter what the temperature is no matter what the pressure is no matter what the motion of the quartz crystal is so that's how we measure time passing by comparing one repetitive thing to another repetitive thing of course this also gives us the feeling that time is passing because within our own bodies there are repetitive predictable cyclic things our hearts beat a certain number of times we breathe at a certain rate our central nervous system has signals firing and going back and forth that's what makes us believe the time is passing and allows us to more or less say yeah it's been about five minutes since that thing happened or gee isn't this lecture over yet it feels like a lot longer than an hour sometimes of course we think the time is passing faster or slower that's because the rhythms in our body are messing with us a quartz crystal is really very very predictable compared to the rotation of the earth our heart beats and our pulses and our breathing is not quite so predictable nevertheless it can never it can be pretty good good enough for certain jobs there's a wonderful story and by wonderful I don't necessarily mean true I'm not sure if the story is true but it's a really good story so it should be true about the young Galileo Galileo is a obviously crucial figure in the history of physics he did many wonderful things late in life he got in trouble with the church early in life he was going to church he was having you know being a good Pisan teenager he grew up in the city of Pisa and he went to church in the Cathedral right next to the Leaning Tower of Pisa but nevertheless the sermon every Sunday isn't always as good as the sermon every other Sunday sometimes you get bored Galileo looked at the chandelier this is the actual chandelier that still hangs in the cathedral at Pisa and it was rocking back and forth and what Galileo noticed this is why he's Galileo and we're not we would say oh yes that's pretty chandelier going back and forth what Galileo notices you know some days the chandelier is really rocking other days it's only moving a little bit but it seems that when it's really moving a lot it also moves faster it's moving physically more meters per second even though he wouldn't have thought in those terms so he made a guess he said I bet that no matter how broad the oscillation of the pendulum it takes the same amount of time to go back and forth so if it's only rocking back a little bit it's also moving slowly if it needs to rock a lot it needs to move faster so that it completes one oscillation in exactly the same amount of time of course the question is how do you know how do you test that and what Galileo did was he used his own pulse so while the priest was talking about whatever the priest was talking about that day I don't want to speculate Galilei was going one 1,000 two 1,000 with his pulse and he noticed that on consecutive Sundays it was true the chandelier had the same period going back and forth so there's a repetitive predictable thing his pulse and another repetitive predictable thing that chandelier he was measuring the passage of time that principle is still used in grandfather clocks or any other system where we use pendulums going back and forth to measure time this is in fact the physicists love this system of a small pendulum going back and forth they call it the simple harmonic oscillator the first thing you learn in a physics class you take in college other things of course do not repeat themselves time passes in ways which are not cyclic they only happens one way human beings change some people start out relatively skinny and they puff up a little bit as time goes on but they remain the King throughout austria-hungary might shrink a little bit after World War one compared to when it was starting a World War one these are not cyclic phenomena you never go back to where you started your taste and architecture might change technology might change biological systems change we grow older with time the species themselves change there's biological evolution these are sort of the interesting aspects of time not that things happen over and over again predictably but some things only happen in one direction then they don't go back so what is the relationship between these two kinds of things the fact that some things the way that systems we use to measure time need to be predictable and have the same thing happened the same number of times whereas things around us in our everyday lives change in ways that you never come back they are irreversible changes well before answering it let me point out that the universe itself has this property that it is changing in time in an irreversible way things within the universe like stars very definite lifecycles stars start from clouds of gas and dust they condense they burn their nuclear fuel they puff up to become you know supernovae or collapse to become smaller stars but the universe that contains us all also changes in a very definite direction in time we used to wonder more than 10 years ago in fact we didn't know what the future of the universe was going to be like now we more or less have a leading candidate for what the future is going to be like it is not going to be one in which the universe re collapses we look around us in Edwin Hubble as I'll mention later in the talk discover that the universe is expanding with time the galaxies are moving a pardon already that's just a remarkable fact not only about the universe but about time the universe is not steady even though the universe is large and certainly older than any of us it is not constant through time it's changing somehow so if the universe is expanding if galaxies are getting further and further apart we can play that movie backwards they must have been closer and closer together at earlier times as soon as you know how fast the galaxies are expanding you can do a rough back-of-the-envelope calculation and say well when were they all on top of each other and the answer is roughly 10 billion years ago these days cosmologists are able to really pinpoint that and say it's 13.7 billion years ago all the galaxies we see around us were on top of each other we call that moment the Big Bang there's a lot we don't know about the Big Bang we be talking about that also but one thing we do know is that near the Big Bang in time everything was on top of everything else the density of stuff in the universe was amazingly high much higher than anything we can reproduce here on earth today and it's been expanding and cooling ever since then things have been moving apart from each other the universe is becoming more dilute and quieter and colder for the last 13.7 billion years and it will continue on in the future we believe the universe is not going to wreak elapsed this expansion is going to continue in perpetuity so this already should be a little bit puzzling because 13.7 billion years seems like a lot of years but then we have infinity years in front of us so we're really in the very very beginning moments of the history of the universe if this is true it's not cyclic we're not going back now through this are there has been developed an explanation for why things change and don't go back that scientists like to refer to and that's entropy entropy is a quality that different systems have that basically roughly speaking says how disorderly a system is so if you have a desk or if you have your office and all the papers are nicely stacked then the entropy is low orderly nice organized means low entropy and you know that if you have a nice organized office and you go away and things happen and you don't try to fight it it will become more disorderly with time the entropy will go up that has been elevated to the status of a law of nature the second law of thermodynamics the second law of thermodynamics says that if you have a system it has an entropy and you leave that system alone you don't touch it you don't try to clean things up the entropy will only ever increase entropy only ever goes up or it could remain constant but it will never spontaneously go down so if you leave your office looking like this and you go away for a few weeks you'll let anyone come in and you come back to your office it's not going to be arranged nice and neatly in orderly piles that's not going to happen this is not by the way a randomly chosen office this is the office of Alan Guth a famous cosmologists at the Massachusetts Institute of Technology he's famous for two things number one he invented the inflationary universe scenario of the beginning of our observable universe and number two he won a contest sponsored by the Boston Globe for the messiest office in the Greater Boston metropolitan area as I think that he's just trying to demonstrate the second law of thermodynamics but there's all these examples of the second law of thermodynamics of course an office getting messy is one of them an egg breaking is another one a broken egg has a higher entropy than an unbroken egg if you take a cup of coffee and you pour milk into it the entropy goes up as the milk spreads into your coffee it's easy to get the milk to spread into the coffee you could wait a long time it would never unspread out of your coffee so what is going on here why is it that entropy is going up and what does that mean in terms of the arrow of time well if we look at the deep down laws of physics we don't see entropy we don't see the arrow of time the deep down laws of physics work both ways but it was literally Boltzmann in the 1870s who finally figured out what entropy really means we can say yet has something to do with order disorder but what does that really mean well here's an example of irreversibility the growth of entropy in a system if you take billiard balls that have been nicely wrapped in a nice triangular shape and there's a cue ball the idea that you could take the cue ball smack it into the billiard balls and they would scatter into some random configuration that makes perfect sense to us that's how time evolves the idea that you could take billiard balls that are randomly scattered hit the cue ball into them and they could all nicely arrange themselves as if they were racked that's not going to happen that's what's very very unlikely that is an example of entropy in the second law at work it's not the perfect example because when you hit that cue ball it's not a closed system anymore you're prodding it a little bit but it does uncover what is going on at the deeper level what is going on is that what entropy is really measuring is the number of ways you can rearrange things it's measuring how delicately things are ordered the point is that when you rack the balls on the billiard ball table there's only one way to Rack them like that but there's lots of ways to be scattered so when you break the balls scatter across the table they look like a mess if someone else broke the next time you're playing a game they would be a different mess but they would still basically look like a mess there's one way to be rack there's a lot of messes there's very small numbers of ways to be low entropy there are a lot of ways to be high entropy the puzzle is that the deep down laws of physics don't know about that if you have a small number of moving parts you don't see the arrow of time if you only had two billiard balls what could possibly happen they could bump into each other and they could scatter off but you wouldn't know before or after which was which there's no such thing as a disorderly arrangement of two billiard balls and indeed at a very technical level you can dig down to the laws of physics and say did the laws of physics point in any direction of time and the answer is no there's no law of physics ever since Isaac Newton whether its Newtonian mechanics or Maxwell's electromagnetism or general relativity or super string theory they all have the property that the deep-down fundamental level there so in particular if you took a movie of these two billiard balls smacking into each other scattering off then I played that movie backwards you wouldn't be able to tell me which way was forward which way was backward if I play the movie backwards of all the balls being scattered from a rack when there's many balls on the table then you know that if they're arranging themselves spontaneously someone is tricking me with the movie but with two balls you would never know that laws of physics at the deep-down level item-by-item particle by particle work equally well forward in time or backwards in time so here is Boltzmann this is Ludwig Boltzmann he passed away was a sad story in 1906 he committed suicide but he was had already had a tremendous career as a scientist and this is his tombstone and his tombstone unlike most other tombstones has an equation on it so that that's when you know you've hit the big time as a scientist when your tombstone has it's not even any like subscript you know there's no gloss or explanation it's like you're supposed to know what this tombstone is telling you what it's telling you is what entropy really means Boltzmann says that what the entropy really is is a way to count the number of ways we can take the parts that make up some system and rearrange them so that you wouldn't notice from a macroscopic point of view it's exactly a way to figure out how many arrangements there are that in some sense it doesn't matter what the arrangement is so an example is a glass of water with ice cubes in it well what happens in terms of ice cubes and the arrow of time Ice Cube's melt you see ice cubes melting in the water just becoming slightly cooler that's a movie that makes sense someone shows you a movie of a glass of water where ice cubes are suddenly forming you know they're playing it backwards that's an irreversible process so what's going on according to Boltzmann is that when you have the melted glass with water in it melted ice cubes in the glass of water there are many many ways I could rearrange the individual atoms within that glass of water so that you wouldn't notice when you look at a glass of water you say I know how much water there is I know it's temperature things like that I certainly don't know the position and the velocity of every single atom or molecule in the glass of water I don't need to know it but if there's an ice cube well then there's water molecules in the ice and there's water molecules in the liquid water and they're different they have a different temperature I could not switch the ice molecules for the water molecules you know they're made of the same stuff because they have different amounts of energy there are fewer rearrangements I could do when there's a separate cube of ice and liquid water than there are when it's all liquid water so from that point of view it's perfectly clear why entropy tends to go up entropy counts the number of ways that a system can be rearranged and therefore if you start in a state that has a low entropy it will just naturally move towards higher and higher entropy because there are more ways to be high entropy than to be low entropy this is a tremendous triumph at a time in the 1870s when physicists as a field didn't really like the idea of atoms they were skeptical about that Boltzmann and some other people Maxwell in England and Gibbs in the United States were champy championing the idea that when you see gases and things like that we should think about them in terms of atoms because we can explain things better Boltzmann's formula for entropy was a classic example of that he said as long as you think of entropy as the number of ways you could rearrange a system that you'll have to work hard to derive the second law of thermodynamics if you start in a low entropy state there's not that many ways to be there you will naturally go to higher entropy well that's good as far as it goes but it leaves us with a mystery entropy increases because there are more ways to be high entropy and low entropy that explains perfectly well why the entropy will be bigger tomorrow than it is today but the second law says more than that it says that it was lower yesterday than it is today how do you explain that and Boltzmann and many of his colleagues at the time tore their hair out and you saw he had a lot of hair trying to understand why the second law works throughout time why can we possibly understand how the entropy was lower yesterday well we know the answer I can tell you the answer now it's because it was even lower the day before that and that goes all the way back to the Big Bang the reason why entropy has been increasing throughout the entire history of the universe is because it started at the very first moment in exquisitely low entropy state that is a fact that is not something we can derive from the second law itself it is the input it is an assumption we sometimes call this the past hypothesis our universe around us started in an exquisitely finely tuned delicate state of extremely low entropy and everything since then for the past 13.7 billion years has been a gradual increase in entropy and thank goodness all of our lives all that we experience in our lives is because we live in the aftermath of this very special event but it still leaves a mystery why was the entropy so low in the early universe Boltzmann could not tell us that that is a job for cosmology what was the early universe like and why was it like that right now we do not know the answer so I'm here to tell you some of the guesses that we've had for what might be going on but more to impress upon you that this explanation of why Ice Cube's melt in a glass of water has a part that we don't understand the traces all the way back to the origin of the universe so here's the universe this is a picture of the universe taken with the Hubble Space Telescope this is the Hubble Ultra Deep Field someday I will work up enough courage to give an hour-long talk that consists of putting this picture up and then standing in silence for an hour as we all contemplate this picture we live in a big universe we live in a galaxy the Milky Way with approximately a hundred billion stars the universe contains approximately 100 billion galaxies so this picture is what you get when you take a sufficiently powerful telescope and point it at a blank region of the sky a region where you didn't know there was anything there it will see photons coming from billions of light-years away that gradually make this beautiful picture where almost every one of these little blobs is another galaxy with about a hundred billion stars so you can imagine this is why Hubble is looking pretty smug there he's the one who figured out that all these little blotches are separate galaxies before Hubble came along we didn't know whether the blotches that we saw in the sky were just clouds of gas and dust within our galaxy or separate galaxies on their own right so one tupple made his measurements to figure out the distance to some of these galaxies our view of how big the universe was increased by an enormous amount the good news is that it doesn't really matter what direction you point your camera in you're going to get more or less the same picture even though the universe is extremely big full of galaxies it has the same number of galaxies roughly speaking over here and over there it's very uniform on large scale so you imagine looking out at night in all directions you would see a hundred billion galaxies spread uniformly through the universe and it's getting bigger I already told you that in the early universe things were squeezed together more or less on top of each other in the future they'll be stretched apart so as the universe expands what does that mean that means that the distance between two galaxies increases and increases with time the galaxies themselves do not increase as the universe expands because things that are held together by any kind of force are separate from the expansion of the universe distant galaxies which are separate from each other it's moving universe expands but galaxies themselves do not expand the planet does not expand and Adam does not expand you are not expanding or if you're expanding it's not because of the expansion of the universe there are other effects one thing that does expand is a wave of light a photon or a light wave traveling through the universe does in fact is not held together by any force it is stretched along with the expansion of space so short wavelength high-energy photons of light gets stretched into long wavelengths lower energy photons that go from being blue low wavelength to being red long wavelength that's called the redshift of the universe and that's associated with the cooling down that we also see the temperature of the universe was incredibly hot at early times it's very very low now three degrees above absolute zero we're about 300 degrees above absolute zero outside right now so if you trace it all the way back I cheat a little bit on the slides at 14 billion it's really closer to 13.7 billion years ago our knowledge gives out we don't know what happened 13.7 billion years ago we know that one second after 13.7 billion years ago we can describe the early universe very well but at T equals zero itself were a little bit lost another thing we're lost about is right now the universe is not only expanding its expanding stir and faster this is a recent discovery in 1998 cosmologists finally had brought together the technology necessary to not only look at the velocity of distant galaxies but to look at what they thought was going to be their deceleration if you think about it galaxies are moving apart with the expanding universe but they're pulling on each other with their mutual gravitational attraction so you would have expected that the universe is expanding but it's expanding more and more slowly as time goes on so in fact there was a collaboration scientific collaboration called the high redshift supernovae team measuring the deceleration of the universe they went out the long story worth many talks in its own right and they came back and said actually guys the universe is accelerating if you look at a distant galaxy with a certain velocity right now you measure its velocity very precisely come back a billion years later that velocity will be increased individual galaxies are moving away from us faster and faster why is that we actually this is this is something where it's easy to slip into a language which says we have no idea whatsoever but that's not true we actually have some pretty good ideas they're not done yet we don't have the full picture but the basic idea is there's something called dark energy and dark energy is just a stand-in for some form of energy which is persistent as the universe expands so ordinary stuff remember it dilutes away the number of galaxies isn't changing but the density of galaxies goes down because space increases the idea of dark energy is that the energy density stays the same so the amount of energy in a given cubic centimetre doesn't change even as the universe expands and if it doesn't change very much the simplest possible version of that is an energy density that doesn't change at all and we call that specific subset of the dark energy idea vacuum energy so vacuum energy is really just the idea that if you take a cubic centimeter of space and you remove everything from it so this is empty as it can possibly get there's no dark matter there's no ordinary matter there's no photons neutrinos etc and you say ok it's an empty cubic centimeter of space how much energy is there in that empty cubic centimeter and you might guess well it should be 0 there's nothing in there the answer according to Einstein and his followers is actually that's not true it's an open question there's a constant of nature a feature of the laws of physics which gives us the answer to the question how much energy is in empty space and if there's any nonzero answer there's any little tiny bit of energy in empty space Einstein says that causes space to expand and recently the amount of energy in empty space has taken over there's more energy of the vacuum energy form in the universe than there is energy in ordinary particles in in ordinary matter atoms and molecules and in dark matter for that matter and what's going to happen is therefore this dark energy is pushing against the universe causing it to accelerate causing it to expand faster and faster that's what gives us some confidence that it's not going to wreak elapsed because dark energy doesn't go away as far as we know and its effect is to make the universe expand faster and faster there's no returning to the state of the Big Bang there's definitely an arrow of time in the expansion of the universe from the hot dense early times to the cold lonely emptiness of the far future so be glad that you're here in between when things are hospitable so the origin of the arrow of time is fundamentally to be found in cosmology Boltzmann gave us a framework for understanding what it means to have irreversible processes entropy goes up because there are more ways to be high entropy than low entropy but the ultimate fact as to why there was ever low entropy is to be found in the early universe at very early times things were extremely smooth right now they're sort of medium smooth it's true that there are galaxies the galaxies are smoothly spread throughout the universe but there's a lot more stuff inside a galaxy than outside the galaxy so there's a certain lumpiness on medium size or small size scales in the early universe there was essentially no lumpiness this is an image this is a snapshot in the bottom right of the universe at about 380,000 years after the Big Bang this is the Cosmic Microwave Background the leftover radiation from the moment in the history of the universe where it cooled off enough to become transparent earlier than this it was so hot that light kept bouncing into electrons running around in the universe after this point light could just travel all the way to us so these particular images that you see here are photons that have been moving through the universe unimpeded for over 13 billion years only to be unlucky enough to smack into one of our telescopes in this case the W map satellite telescope from NASA you see the tiny little fluctuations there that are actually enhanced by a lot for clarity but these are small differences in temperature of the Cosmic Microwave Background from place to place only one part in a hundred thousand so if you have some square centimeter of space at that time that has a hundred thousand atoms in it the square centimeter next to it is going to have a hundred thousand one or 999,999 sorry 99999 so there were tiny little variations in the density of the universe from place to place but they grow with time because if there were slightly more atoms here than there that region has a slightly stronger gravitational field and pulls more atoms into it so the expansion of the universe acts to turn up the contrast knob through the force of gravity tiny fluctuations become big fluctuations these little ripples you see in the microwave background become stars and planets and galaxies that we see today the universe has become lumpier over the past 14 billion years eventually it's not going to be lumpy anymore the lumpiness that we have now is going to dissipate the stars that we have in our galaxy are going to fall into a giant black hole the center of our galaxy we already know there is a million solar mass black hole at the center the Milky Way galaxy that's not that much million solar masses isn't that much in a hundred billion solar mass galaxies but give it a few more trillion years all those stars are going to fall into that black hole the black holes don't sit there forever black holes evaporate so if you wait a googol years if you wait ten to the 100 more years from now all the black holes will have formed and evaporated there's gonna be nothing left in the universe but empty space so the lumpiness is just a phase we're going through it's the puberty phase of the universe but the ultimate age everything is going to be flat empty dilute cold nothing going on why is it like that this is the problem before telling you why it's like that let me just impress upon you how important this is that it was like that everything that we know about the arrow of time be traced back to the low entropy of the Big Bang what does that mean that means life and death aging biological evolution the fact that you can remember yesterday but you don't remember tomorrow yet the fact that we feel the time is flowing around us there's a whole set of different attributes of what we call the arrow of time but they all have a common explanation they're all originally oriented in the second law of thermodynamics which needs to be explained by invoking the low entropy of the early universe so this picture is a schematic drawing of the earth and the Sun not to scale in real life the Sun is bigger than the earth but the point is it's supposed to get across the fact that if entropy were high if we were not in this weird universe if you were in a maximally disorganized universe we would call that equilibrium everything would be the same everywhere be like the glass of water where the ice cubes had melted everything would be spread out and smooth and then there'd be nowhere to go if entropy is high it would just stay high for ever and ever nothing interesting would ever happen the only reason why anything interesting ever happens in the universe is because we are nowhere near equilibrium because entropy is much lower than it could possibly be and the most important manifestation of that is the Sun in the sky in equilibrium things are smooth and spread out so when you look at the sky we look the same everywhere that's not what we actually see when we look at the sky we have the Sun which is a bright hot spot in a very cold empty sky and it's because of that arrangement that everything around us is possible the earth receives energy from the Sun as you know it also gives off energy back to the universe around it it might not be a merely obvious but the amount of energy we get from the Sun is almost exactly equal to the amount of energy we give out to the universe otherwise you have to be heating up it is heating up by the way just a little bit that's a problem but not a problem for this talk to my approximations we give off the same amount of energy as we receive but not the same amount of entropy it turns out that for every photon we get from the Sun the earth radiates twenty photons into space around it that's 20 times the entropy we were able to gain a tremendous amount of entropy just by processing the energy we get from the Sun we useful energy in a low entropy form from the Sun we give it off in a high entropy useless form to the rest of the universe and that powers the plants that fed the dinosaurs that died and turning to oil and lets us get in our car move around everything that happens here on earth is because the Sun is a hot spot in a cold sky that's altom utley a low entropy configuration which is allowed because the Big Bang had a low entropy and this includes this whole chain of logic includes not only biological scientific sounding things it includes things like causality cause-and-effect the idea that we have a free will that lets us make choices about the future this is all because of the arrow of time if you think about an egg you see the egg broken on a sidewalk somewhere and you ask yourself what might happen over the next 24 hours to that egg that is broken on the sidewalk you say well I don't know lots of things it could sit there and it could it could bake or freeze depending on the temperature someone could go clean it up a dog could come by and eat the egg or something that says a lot of different possible things but then if you say well what do you think the last 24 hours of the egg were like you were like well probably 24 hours ago it was an unbroken egg you can be much more specific about what could possibly happen to that egg if you think about what happens in the past that in the future why is that because we're making an assumption we're making the past hypothesis we're assuming that we start in an extremely low entropy state if you didn't make that assumption you wouldn't be able to say any different thing about what the past of that egg was like than about the future likewise for any decisions you make in your life our feeling that the past is finished put in the books unruhe triva believable unchangeable is not anywhere to be found in the laws of physics according to laws of physics the past and the future are equally real equally determined etc etc the difference in the real world is that the past is hamstrung by the fact that it has to connect to a low entropy boundary condition so the fact that you think you can change your mind about what to have for dinner tomorrow night but not what you did have dinner about last night is because of the arrow of time so to understand it we need to understand the Big Bang there's various possibilities and here's where I go from saying things that are true to wondering what might be true from here on in I don't know what is right and what is wrong we're going to explore some different possibilities my own biases will come through very clearly so I'm going to suggest three different possible scenarios all of them are pretty much on the table one is that the Big Bang just was special it was the beginning of the universe by the way this wouldn't have been an option to Ludwig Boltzmann in the 1870s he didn't know about the expanding universe he did know about the Big Bang etc but these days in fact since we figured out the universe is expanding in the 1920s it seems very clear to us that okay there's a place where we can hide all of the mystery of the low entropy of the early universe there's the Big Bang we don't know what happened at the Big Bang Stephen Hawking might know or someone else might know but maybe there's just a law maybes an extra principle of physics that says at early times the entropy was extremely low in fact there's a specific implementation of this idea there's related to the idea of inflation I talked to you just earlier about Alan Guth cosmologists at MIT suggested a way to help explain why the universe around us is so smooth and homogeneous remember we have this tens of billions of light years across universe with 100 billion galaxies in it that looks more or less the same everywhere that seems very delicately tuned why couldn't be different here than there so what Alan Guth pointed out was he said well imagine there was a very very tiny region in the very early universe that was dominated that was full of an incredibly high temporary form of dark energy remember dark energy doesn't go away as the universe expands it makes the universe accelerate but only very very gently if you had a temporary form of dark energy that was very virulent that an extremely high amount of energy would make the universe accelerate at a tremendous rate so Guth assumed that you had just a really tiny region much much less than a centimeter across dominated by dark energy and that that stretched to a tremendous size and that stretching smooths everything out like pulling at the edges of a wrinkled sheet making it very very smooth and then by some miracle the dark energy went away it turned into ordinary matter and radiation and we see the aftermath of that as the Big Bang this idea is inflation and it has been subscribed to has been the and paradigm in cosmology since Guth came up with it in the early 1980s it solves the problem of why the universe looks so smooth the problem is as far as entropy is concerned it's a disaster it does not tell us why the early universe had a low entropy except by the logic that it had a low entropy because the entropy was even lower before that that little patch from which the universe came had an extraordinarily low entropy and the way to think about that is everything you see around us today in this room the rest of the earth the galaxies the hundred billion galaxies around us all that stuff was in that little patch it was it was implicit there but the degrees of freedom the physical system from which we came were very very delicately arranged so that everything we see around us could fit smoothly into that little patch there aren't that many arrangements of the degrees of freedom of the universe that would make that happen that's an extremely low entropy finely tuned configuration so inflation might be right I'm the kind of person who thinks inflation has a lot going forward it might be on the right track but it doesn't tell us why the early universe had a low entropy it demands that we understand better than we do now why the early universe was like fat the next idea is a cool one and it actually does go back to Boltzmann and says well maybe the universe is just the kind of thing that happens from time to time it's a very appealing idea it sort of removes us from any specialness but it doesn't work so I want to tell you even before I go into what it means is it doesn't work so it's not this is the one I think I can say with some confidence is not right but the reason why it's interesting is because the words that Boltzmann uses when he thinks about this idea are the same words that cosmologists use today when they talk about the multiverse when they talk about the entropic principle when they talk about the idea that the universe we see around us is just a tiny little part of a much bigger system we think that's all avant-garde and new stuff Boltzmann was talking about that over a hundred years ago so here's a quote from Boltzmann going back to 1895 would he imagine remember he didn't know general relativity quantum mechanics etc he thought Newton was right he imagined a universe full of particles like on the diagram on the left here and he says look if you wait long enough since I'm Boltzmann and I know what entropy is the entropy can in fact occasionally go down even though the second law says it goes up because if you wait long enough there will be fluke fluctuations it will just so happen that all the particles will bunch up in one region of occasionally lower entropy and they will move back they will relax and Boltzmann says maybe that's the universe maybe that is everything around us so there must be that in the universe which is in thermal equilibrium as a whole and therefore dead so he's saying by the way life could not exist if we were in thermal equilibrium here in they're relatively small regions the size of our galaxy which we call worlds which during the relatively short time of eons deviate significantly from thermal equilibrium so this is Boltzmann's multiverse it's a huge region infinitely big full of molecules at some temperature they're moving around randomly arranging themselves and occasionally they fluctuate into a very low entropy state we are in the aftermath of that little fluctuation coming back to a high entropy state why are we there rather than the background well because we can live here there's a selection effect we can live in a region of the universe where the entropy is increasing we can't live in thermal equilibrium so he wasn't the first come up with this the first idea I could trace it back to was 50 BC Lucretia is a poet in the early Roman Empire a follower of Epicurus and Democritus just like Boltzmann Lucretia's was an atom missed at a time when most people were not atom as' and he was faced with the same kinds of problems he was trying to explain how we could get all this order and all this structure that we see around us in the universe if it's really just atoms bumping into each other so he says surely the atoms did not hold counsel assigning order to each flexing their keen minds with questions of place in motion and who goes where instead they jumbled and shuffled in many ways in the course of endless time they're buffeted driven along chancing upon all motions combinations at last they fall into such an arrangement as would create this universe it's exactly Boltzmann's idea Lucretia's had fewer equations and Boltzmann did but he's attacking the same problem how can the mindless shuffling of random atoms create all this wonderful stuff we see around us and he says well if you wait long enough it's just the kind of thing that's going to happen from time to time this scenario the Boltzmann Lucretia scenario for our low entropy early universe does not work why doesn't it work was pointed at least by Arthur Eddington in the 1930s it might have been point out before that but I haven't been able to find a reference and the problem is that if you are Boltzmann and you have his equations you know that it is true that fluctuations happen if you're at maximum entropy equilibrium everything smooth occasionally you'll go to lower entropy but usually you'll go to lower entropy a little bit and then you'll smooth out again it's much much more rare to have a huge fluctuation of entropy than a tiny fluctuation of entropy so Eddington set so the punchline of this is that this scenario that we're a fluctuation in a thing that is usually in thermal equilibrium makes an incredibly strong prediction and that prediction can be compared against data and it comes out false the prediction is that we are the smallest possible fluctuation in entropy that is allowed by then you tell me what it needs to be allowed by so if you put any condition say such that human beings exist such that the earth exists Eddington says that the correct anthropic criterion is such that there are mathematical physicists and you make a prediction that you are the smallest possible fluctuation entropy consistent with that criterion so he says it is practically certain that at any assigned date the universe will be almost in a state of maximum disorganization a universe containing mathematical physicists blah blah blah will be in a state of maximum disorganization which is not inconsistent with the existence of such creatures so if I really believe this scenario and I said well I know that you guys are all here right now I know that I'm here in this room I can see it so I'm going to trust my senses believe this room is here and I believe this scenario and that includes by the way all my memories all my records all my thoughts about the laws of physics and laws of nature and logic I say what do I predict what I predict is that when I open the door and go outside will be thermal equilibrium everything will be exactly the same there'll be no cars they'll be no Waterloo there'll be no TV stations etc etc that is a prediction that is ruled out all the time and in fact you can go to the reductio ad absurdum and say who needs you and I just need me and in fact who needs me I just need my brain because that's where all the thoughts are going on so this reduces us to the idea of Boltzmann brains that if we were really in a universe which is mostly in thermal equilibrium with occasional fluctuation almost all of the conscious thinking observers in that large multiverse would be disembodied brains that it randomly fluctuated out of the thermal background and then came come back it's much much easier to do that than to create the whole universe and have the brain be a small comfortable part of that that is the argument that says this Boltzmann Lucretia scenario cannot be right our universe is not a low entropy fluctuation within a high entropy background so this is true I believe this you should believe it however not everyone believes it so this picture here came from an article in New York Times about this scenario and I was quoted in the article and afterward I received a little piece of fan mail this is from George winged ten years old who was very agitated by this article and I will read it's hard to translate George is very excited so I will translate the art what his letter says I don't know if you exist but I do I do not agree with your article I do not believe that mumbo-jumbo if you do well it's a disturbing thought but I know how to deal with it I will not let the world disappear under my nose but if you do I can't say I'm sorry sincerely a ten-year-old who knows a little more than some people George wing PS some people have a little too much time so I'm looking forward he's ten years old ten years from now and George is applying to Caltech I'm going to blackball his application because I don't want any people like that giving me attitude it's difficult to accept because we talked about Boltzmann brains it sounds like we're suggesting that we really could be a brain popping out of nothingness and it is true I am suggesting that but the point is that the alternative being suggested by the scenarios that the universe is popping out of nothingness and that's even less likely than a brain popping out of like out of nothingness therefore the scenario is probably not right so if those two scenarios are not right I mean the first one could very well be right it could just be there's an extra law of physics I don't like it it seems like a cop-out to me it seems like instead of answering the question we're saying yeah that's a good question and then moving on to something else and that's a traditional cosmological attitude toward the problem I want to do better than that the second attempt is at least a attempt to do better than that but we also have the third possibility which is that the Big Bang is not the beginning of the universe but it's also not just thermal equilibrium something existed before the Big Bang that explains where we came from after all you see eggs in the universe even though eggs are low entropy you could ask yourself well why are there any eggs in the universe if all this babble that things want to be high entropy is right why are there eggs and the answer is the egg is not a closed system there was a chicken that helped create the egg so could there be a universal chicken could there be something from which our universe was created in its relatively low entropy state and and keeping you'll notice that the chicken is not an intelligent designer the chicken is not like putting together the proteins in the egg it happens through very naturalistic processes we want to know are there equations or the dynamical rules governing stuff in the universe from which there could have been a prior state out of which our universe came now you've probably heard that in fact the Big Bang is the beginning of space and time this is a common thing that cosmologists tell people all the time they say you know they said a Big Bang what is going on what happened before the Big Bang and you're told by very very well-meaning and highly educated cosmologists there was no such thing as before the Big Bang the Big Bang was the beginning of everything there was no prior moment both space and time came into existence at that moment if you've gone this long your life without hearing that before the Big Bang is like being north of the North Pole then it congratulations to you because that's what you hear most of the time but it might not be true in fact it's very very inaccurate to say that we have a good reason to think that that is true that is an extrapolation beyond where the currently known and understood laws of physics allow us to go what is actually true is that as you go back to the early universe using laws of physics as we know them you don't hit a moment of time before which there was no time you hit a point where your equations aren't good anymore Einstein here's a picture of Einstein that you usually see pictures of Einstein with the gray hair and the rumpled sweatshirt and so forth late in life but when Einstein was doing his great stuff special relativity general TV things like that in the early you know this is 1912 I believe he was a sharp-dressed young man someone was combing his hair I don't know what was going on but he was clearly a smart guy a little bit on the ball figured out how space and time worked but his theory general relativity which explains gravity as the curvature of space-time is incomplete we know it can't be right because it's inconsistent with itself general relativity predicts its own downfall because it predicts there should be places and times where the equations of general relativity don't apply those moments are singularities places where things that should be finite numbers become infinite and the Big Bang is one of them so the correct thing to say about the Big Bang is not that there was no time before it is that our current understanding of the laws of physics gives out at that moment in time we need to think a little bit harder we should be open-minded it could have been the beginning or it could have been a phase through which the universe goes so the way this I like to think about this asking well what kind of configuration could the universe have come from is to think about the second law of thermodynamics the second law says well entropy goes up as we go from unnatural States to natural States we don't understand entropy when gravity is concerned because we don't know the space of all the possible states that's problem of quantum gravity we don't know how to marry together quantum mechanics and general relativity yet so one possible attitude is to throw up your hands and say I don't know what to do another one is to say that even if I don't know what entropy is I know what's happening to the universe I know where it's going I know it's going to look like in the future and if it's true that entropy is going up I can infer what a high entropy state looks like just by asking what the universe will in fact look like billions of years into the future well we know what's going to happen it's going to look like empty space the universe is becoming more more dilute things are emptying out distant galaxies are receding away from us eventually the stars will collapse into black holes the black holes will evaporate we'll be left with nothing but empty space but as we know we think that empty space now has energy in it vacuum energy a tiny tiny fraction of an ERG in every cubic centimeter of space and what that means is that if empty space has energy in it it also has a temperature that even if you're in perfectly empty space if you sit there with a thermometer you will detect stuff around you your thermometer will detect a non-zero temperature that means that there are fluctuations that empty space is not a quiet place and this is just one of the miracles of quantum mechanics that you can't pin down empty space to perfect quietude there's always little ripples there's always little fluctuations it could be that one such fluctuation makes a new universe so the natural place for the universe to be is in a high entropy state that's empty space nothing there but vacuum energy but these fluctuations could conspire in a special way the number one space-time itself fluctuates into a funny shape where you have a throat connecting a bulbous region of space-time and then the fields that make up space could fluctuate inside to make guess what a super dense region of dark energy that is ready to undergo inflation so in other words we have a mechanism which we can least draw pictures there's some equations also but this is a not fully finished story yet but it could be that you go from empty space very very quiet to creating a little part of space that is disconnected branches off and becomes a baby universe this little pack this little tiny little thing according to Alan booths theory will expand and become a universe just like ours so the reason why this is an interesting story is because it can explain why you go from emptiness to a universe that is alive like ours you start with a universe that is perfectly quiet no arrow of time you're in thermal equilibrium it's empty space nothing's going on but every now and then if you start in the middle of this picture and move to the right what's happening are that baby universes are created very very very rarely but they grow up and they have their own entropy and the total entropy of the universe increases through that mechanism but our universe it could be one of those baby universes but not the only one we could have our own babies in the future that we could have siblings next to us furthermore if again you start in the middle and go to the left the same story will be told so according to this picture there's the reason why the universe has entropy is increasing around us because the universe can always increase in entropy there is no configuration it could be in where the entropy would max out and stop moving you thought you thought you were there with empty space but even empty space can make more entry by making more universes and the overall multiverse that has all this stuff going on in it is actually symmetric the past looks like the future these separate universes far far in the past have an arrow of time that is pointing in the opposite direction the bad news is that we can't talk to them we can't really converse with them we don't know that they're there by any experimental means but it's a story that fits together the question is is the story right I don't know so what I did is I wrote a book about it writing a book is sort of the last refuge of a scoundrel as far as science is concerned if I really knew the answer I'd write a paper and we refereed etc but the answer is not the important thing here the family to emphasize is not my particular favorite scenario for answering this well what I want to emphasize this is a really important puzzle that connects things that goes on in our in our kitchen to the very early universe and it's puzzle we are not close to having the right answer to yet so what we know is that entropy is responsible for the arrow of time all those different manifestations of the difference between the past in the future are reflections of the second law of thermodynamics all that stuff reason why entropy is increasing is because it was very low to start with in the early universe and number three we don't know why it was low so one of the things we know is that we do not know why the entropy was very low in the early universe the speculative part is were they initial conditions were they all special were they all by themselves or did we come from something else that we come from a bigger universe I don't know the answer we're working on it hard I think we're close to the point in time when science is finally got its act together and we can approach these questions scientifically and I'm hoping that at the 20th anniversary of the premier Institute ten years from now you'll come back and I'll give you the answer to all these questions thank you

Channel: TVO

Views: 528,221

Rating: 4.6279531 out of 5

Keywords: TVO, TVOntario, TVOKids, polka, dot, door, polkaroo, education, public, television, Elwy, Yost, Steve, Paikin, big, back, yard, ideas, Canada, Big Bang, Big Ideas, entropy, multiverse, arrow of time

Id: rEr-t17m2Fo

Length: 55min 35sec (3335 seconds)

Published: Mon Dec 13 2010

Reddit Comments

[u/brandonmat]

I saw this live at the Perimeter Institute. It was a great talk and was very interesting!